James is expected to miss a month, NFL Media Insider Ian Rapoport reported, per a source informed of the prognosis. The third-year back suffered the injury while participating in a blitz pick up drill.

That leaves just two healthy backs (who are locks to make the team) for the Niners -- Frank Gore and second-round pick Carlos Hyde. Marcus Lattimore is currently on the non-football injury list as he attempts to recover from serious knee injuries in college.

James had just 12 rushes last year, but also returned kicks and punts for the team. He reportedly was shopped by the 49ers in April.